This invention relates to fuel cell power plants, and more particularly to delivery and control of hydrogen-rich fuel to a fuel cell stack assembly. More particularly still, the invention relates to fuel control for fuel-processor steam generation in a low temperature fuel cell power plant.
Most fuel cell power plants rely on a supply of hydrogen-rich fuel to the anode of a fuel cell stack assembly (CSA) to provide a reducing agent for electrochemical reaction with an oxidant to provide electrical energy, and the by-products, water and heat. The oxidant, which may be air, is supplied to the cathode of the CSA. The anode and cathode are separated by an electrolyte that defines the type of CSA. One common electrolyte is, and has been, phosphoric acid. Phosphoric acid fuel cells tend to operate at relatively high temperatures, resulting in attendant advantages and disadvantages. Another more recent fuel cell is the PEM cell, which employs a polymer electrolyte membrane, or proton exchange membrane, (PEM) as the electrolyte. The PEM fuel cell operates at temperatures (and sometimes pressures) that are well below those of phosphoric acid cells, typically being at temperatures less than the boiling point of water, resulting in a number of advantages that contribute to the growing interest in and application of, this fuel cell. On the other hand, the reduced temperature of operation of a PEM fuel cell also creates some additional challenges.
As is the case for many fuel cell power plants, some source of raw hydrocarbon feedstock is processed within the power plant to provide the requisite hydrogen-rich flow of fuel reactant, or reducing agent, to the anode. That processing is done by a fuel processing system (FPS) that tends to segregate the hydrogen from the carbon and/or oxygen constituents of the hydrocarbon feedstock in any of several known processes. Common to most of those processes is the use of a reformer, with the additional inclusion of a shift converter and possibly a selective oxidizer also being common. The reformation of the hydrocarbon feedstock normally employs a reforming reaction exemplified by: CH4+H2Oxe2x86x923H2+CO, and a subsequent shift conversion reaction exemplified by: CO+H2Oxe2x86x92H2+CO2. The reformation may be performed by various types of reformers, such as, for example, a catalytic steam reformer (CSR) or an autothermal reformer (ATR), as discussed in greater detail in U.S. Pat. No. 6,120,923. The reaction in an CSR is endothermic and requires the addition of heat, whereas the reaction in an ATR is exothermic and does not require the separate addition of heat. Although the type of reformer may vary as a function of the feedstock to be reformed, they have the common requirement that water (H2O), heated to steam, is used in the reaction. Moreover, the reactions are facilitated by the enthalpy of the steam and, at least for the CSR, may require supplemental heat. For the reformer of each particular fuel supply system, there is a corresponding optimal steam-to-carbon ratio for the production of the H2 needed by the fuel cell at various operating/load conditions.
As noted, phosphoric acid fuel cell systems operated at higher temperatures, and thus typically had sufficient heat reserves to provide the necessary steam for the reformation reaction under all conditions. On the other hand, for fuel cells operating at lower temperatures, such as PEM cells, it has been necessary to provide supplemental heat sources to raise water to steam for the reformation reaction. One technique, typified by the aforementioned U.S. Pat. No. 6,120,923, employs unused fuel reactant gas, exiting as exhaust effluent from the anode of the CSA, to fuel a burner and/or boiler to provide the requisite steam and heat. Typically, the flow of anode exhaust effluent fuel used for supplemental heat was adjusted as a function of the electrical loading of the system. Although this allowed the fuel flow to track load demand to some extent, it did not necessarily provide the steam enthalpy needed for efficient transient response, both increases (up) and decreases (down), under all anticipated conditions.
It is desirable to have the CSA closely follow the load, such that the power plant is always operating at, or near, the level required by the loads. Such load-following operation allows the power plant system to be more efficiently sized and operated, while assuring the requisite power for changing load demands. Not only is it desirable to have sufficient steam to process sufficient fuel for the CSA during load increases, but it is also desirable that the steam pressure not be excessive, for both economic and structural reasons, during load decreases. This, then, requires a balanced supply of steam. However, as discussed above, the present management of thermal energy in a low temperature fuel cell power plant is typically not sufficient to provide the responsiveness needed in the fuel reformation process to accommodate the changes in hydrogen fuel demands at the anode of the CSA needed to efficiently support load-following operation.
Accordingly, it is a principal object of the invention to provide an arrangement for managing thermal energy in a low temperature fuel cell power plant to provide the responsiveness needed in the fuel reformation process to accommodate changes in hydrogen fuel demands needed to support load-following operation.
It is a further object of the invention to provide an arrangement for assuring sufficient, and balanced, thermal energy for the fuel reformation process to efficiently respond to load-following operation of the fuel cell.
It is an even further object of the invention to provide an arrangement for regulating fuel flow to the anode of a fuel cell to assure a balanced flow of effluent therefrom for supporting a balanced supply of steam and possible other thermal needs of the fuel reformer to support on-demand operation of the fuel cell.
The present invention is a control arrangement, in a fuel cell power plant having a low temperature fuel cell stack assembly (CSA), a fuel processing system that requires steam, and means for providing steam for the fuel processing system, for regulating fuel flow to the fuel processing system to provide sufficient fuel to the fuel cell stack for on-demand operation and to raise sufficient steam for the fuel processing system. The fuel processing system, using process steam, converts a hydrocarbon feedstock to a hydrogen-rich fuel supply for the anode of the CSA. The hydrogen-containing exhaust effluent from the anode is used to provide thermal energy to raise the process steam for the fuel processing system. The flow of hydrocarbon feedstock is controlled to maintain the enthalpy of the supply of process steam so as to provide the hydrogen-rich fuel supply needed to rapidly respond to increases and decreases in electrical demand on the CSA without exceeding steam pressure limits.
Fuel cell load current is monitored to measure electrical demand and provide a basic control signal to regulate fuel flow to the steam processing system, and thus also to the anode of the CSA. A parameter of the steam, such as pressure or temperature, is monitored as a measure of the enthalpy of the steam supply available for the conversion of the hydrocarbon feed stock to the hydrogen-rich fuel supply required by the anode of the CSA, and provides a correction, or compensation, signal to the control signal derived from the load current signal, to assure a sufficient, balanced supply of steam under changing load demands on the CSA within steam pressure limits during up and down transients. Those steam pressure limits are typically determined by the structural limits of the system relative to high-pressure limits, and by minimum operating requirements relative to low-pressure limits, and may differ from system to system. The actual fuel flow is monitored and provides a feedback signal to the fuel flow control signal. Control of fuel flow may be via regulation of a flow valve, a blower, or the like. The control arrangement includes those elements necessary for the desired response, and may be implemented in various configurations capable of providing the requisite functions. Outer and inner control loops, each having one or more algorithms, typically using proportional/integral (PI) control, are preferably used.
The fuel processing system includes a reformer, which may typically be an autothermal reformer (ATR) or a catalytic steam reformer (CSR), depending in part on the hydrocarbon feedstock and other factors. In the event a CSR is used, a temperature indicative of the endothermic reaction conditions in the reformer, such as the temperature of the top tubes in the reformer, may additionally be monitored to provide a temperature signal that is used to further adjust the fuel flow control signal.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.